1. Field of the Invention
The present invention relates to circuits for detecting the position of circuit contacts in switches in railroad signaling or interlocking control circuits and more particularly to a circuit for safely and reliably monitoring the position of fail safe, or vital, circuit contacts without falsely activating a vital circuit.
2. Description of the Prior Art
In railroad signaling and interlocking control circuits, fail safe, or vital, circuit contacts are used for various switching purposes such as the turning off and on of signal lights. Typically four to six (4-6) contacts are configured in a single relay, the relay being used to control one aspect in the circuit. It is necessary to safely monitor these circuit contacts to verify correct circuit operation and to provide evidence of actual operation in the event of an investigation. Presently, two methods are normally used to monitor circuit contacts: (1) a series method; and (2) an isolated potential method.
The series method relies on electrical components, i.e., light emitting diodes, resistors, relays, etc., inserted in series with the circuit contact, the component indicating the current flow through the circuit contact. The contact is monitored based upon the actual circuit current flow, the current flow being indicated by the condition of the component.
Although the electrical component will indicate whether the circuit is active, if there is more than one contact in the circuit, the electrical component cannot identify which contact is open/closed. Furthermore, inserting electrical components in series in these vital circuits can influence the critical wiring characteristics of the vital signal relays, possibly causing the potential for wrong-side signal system failures.
A variation of the series method is accomplished by utilizing a spare circuit contact in each relay to be monitored thereby precluding the risk of distorting the vital circuit and allowing the monitoring of each relay in a series. However, when each contact in the relay must be used, the relay cannot be monitored in this way.
In those cases where there are no spare contacts in the relay, railroads must install additional vital signal repeater relays in order to monitor safely the individual circuit contacts. In such a situation, the contact in question is effectively taken out of the vital circuit and is used to activate a contact in an additional relay which acts in place of the contact in question. The contact in question can thereby be monitored without affecting the vital circuitry. The need for these additional relays causes the railroad much higher cost and rapidly consumes any available space left in the signaling case or housing.
The isolated potential method relies on a separate, isolated energy source used as a local power supply to monitor the contact in question. The monitor source (typically a 12 v DC battery) is connected in line with an active circuit contact, a monitor circuit being used to detect the monitor source thereby indicating the condition of the contact in question. Great care and many precautions must be taken to preclude the possibility of unsafe or hazardous circuit conditions being created as a result of the insertion of this kind of monitor circuit. For example, it must be insured at all times that the monitor source be isolated from the circuit's return in order to prevent the monitor circuit from falsely activating a vital signal circuit (e.g., Red or stop signal being changed to Green or proceed signal) as a result of leaking monitor circuit.
The isolation potential method carries great risk in falsely activating a vital signal circuit. While a totally isolated battery theoretically would permit safe detection of a single contact, there is too much of a risk of ground faults (either elsewhere in the system or as a result of a direct connection to the isolated battery) to install such a monitor system in a signal case or housing safely.